The invention relates to an apparatus for determining the transit time of ultrasonic pulses in a fluid, in particular in a gas, the apparatus comprising first and second spaced apart electro-acoustic converters of which one transmits ultrasonic pulses along the measurement path contained in the fluid while the other receives pulses which have travelled along the measurement path, wherein both converters are connected to a correlation circuit in order to determine the transit times of the pulses.
An apparatus of this kind is used in a method for measuring the speed of flow of a gaseous medium. For this purpose ultrasonic pulses are transmitted along a measurement path contained in the gaseous medium, in a direction having at least a component in common with the direction of flow of the medium. The speed of flow of the medium can be determined by measuring the pulse transit time for ultrasonic pulses travelling in opposite directions along the measurement path, from the transit time differences.
It is important for a troublefree correlation measurement that the sound pressure signals of the transmitter and the receiver are converted into corresponding electrical signals by the converters, which are preferably composite piezoelectric oscillators. A signal of this kind automatically occurs at the output of the receiving converter, the corresponding signal from the transmitting converter must be generated by special means. Such means are for example described in applicants' co-pending patent application having the title "Apparatus for determining the transit times of ultrasonic pulses in a fluid", (Ser. No. 606,250, filed May 2, 1984, now U.S. Pat. No. 4,576,047). The generation of a transmitter signal of this kind at the input of the correlation circuit is also a prerequisite for the present invention and will be described later in this specification.
The transmit time .tau..sub.0 of the ultrasonic signal is derived from the maximum of the cross-correlation function: ##EQU1##
The cross-correlation function is thus the time averaged product of the received signal y(t) and the transmitted signal x(t) which is delayed by the delay time .tau..
Rather than the complicated search for the maximum it is more favorable, from the point of view of the technical circuitry, to seek the null point of dk.sub.xy /d.tau..
Apparatus simplifications are known which are based on the fact that the following expression applies to the maximum of the cross-correlation function with the delay time .tau..sub.0. ##EQU2## y(t) is in this case the time derivative of the signal y(t).
A further technical simplification of the apparatus is possible by making use of the so-called polarity correlation: ##EQU3## The advantage of the polarity correlation in comparison with the analog multiplication of equation (2) lies in the fact that the analog multiplication, which is subjected to drift, is omitted and that the signals can be digitally processed. However, the signal y(t) has to be differentiated with respect to time in both methods.
A filter having the (normed) frequency response ##EQU4## is used for the differentiation. The term in the denominator is undesired but cannot be avoided. Accordingly a compensation has to take place. Another disadvantage of the differentiation lies in the fact that signal processing without non-linearity errors becomes increasingly more difficult, in particular for steep signal flanks (for example rectangular waves).
The object of the present invention is to provide an apparatus of the initially named kind in which one can avoid the differentiation with respect to time.